Microwave power may be used to power a high intensity discharge. The lamp typically comprises a quartz capsule containing an excitable set of gases and chemical dopants. Microwave radiation is then directed at the capsule, causing the fill materials to heat to a plasma state and emit light. The discharge may have high intensity, and because there are no electrodes penetrating the capsule wall, the lamps are referred to as electrodeless high intensity discharge lamps, or electrodeless HID lamps.
Microwave electrodeless HID lamps in the past have been coupled to power sources using termination fixtures which were typically large, bulky shielded coaxial structures. The shielding has been an incumberance to the efficient capture and display of the emitted light.
A novel dual ended excitation scheme was disclosed by Lapatovich in U.S. Pat. No. 5,070,277. Microstrip line conductors were coupled to microwave applicators directed at a tubular capsule. Microwave applicators that have been used with tubular capsules include slow wave helices, end cups, loop applicators and so forth. The dual ended excitation scheme provides substantial flexibility with respect to impedance matching techniques compared with the older termination fixture techniques. Considerable reduction in size and weight of the microwave coupler was achieved. With the reduced size of the applicator, more of the emitted light could be used in an optical system. The coil coupling and strip line patterning structures required an external variable impedance matching means, which can be bulky and expensive. There is then a need for a microwave circuit pattern for electrodeless lamps having balanced power distribution.
Nevertheless, when certain combinations of discharge capsules and power couplers, are implemented with a half wave balun, or a T junction and half wave balun, there is a greater power loss in the longer arm of the half wave balun structure. The power difference is then manifest as a power imbalance in the discharge tube with one end of the tube receiving more power than the other. The power imbalance leads to preferential heating of one end of the discharge capsule and applicator. The additional heat affects lamp performance, lamp longevity, and reliability of the applicator, and transmission line structures. There is then a need for means of overcoming the power imbalance.